A great many reciprocating piston engines and pumps are known in the prior art which utilize a piston pivotably connected to a connecting rod by a wrist pin. High speed internal combustion engines frequently utilize a aluminum piston to minimize weight and bearing load and a steel wrist pin to achieve the necessary strength characteristics. It becomes very important to securely locate the wrist pin relative to the piston so that it is not longitudinally displaced sufficiently to scrape the cylinder wall resulting in scoring and premature engine failure.
A number of methods have been utilized to securely orient the wrist pin to the piston. One such method is to provide an interference fit between the wrist pin and the corresponding bores formed in the piston. While this technique has achieved wide-spread use, a number of problems exist, namely, if the engine is overheated, the piston pin may become loose as a result of differences between the rate of thermal expansion between the aluminum piston and the steel wrist pin. In order to achieve sufficient interference and in order to minimize the likelihood of the relative movement, large interferences resulting in large press loads occur. Press-fitting of the wrist pin into the piston may result in deformation of the piston causing the piston ovalarity or an out-of-round condition which can, if excessive, result in a localized high wear spot on the piston skirt. In order to minimize pin press loads, attempts have been made to heat the piston and/or chill the wrist pin prior to assembly. However, this is time-consuming and a costly process.
Another prior art method of implanting a wrist pin relative to the piston is to press the wrist pin into the connecting rod while allowing the wrist pin to freely rotate relative to the piston. While this method minimizes the thermal expansion problems since the wrist pin and connecting rod will typically be formed of materials having similar thermal expansion rates, deformation of the piston frequently occurs during assembly and production tolerances for a wrist pin and connecting rod corresponding diameters become highly critical.
An alternative method for retaining the wrist pin in a piston is to utilize snap rings located in recesses formed in the piston to limit the axial movement of the wrist pin. This type of design is frequently referred to as a full-floating piston since the wrist pin is free to rotate relative to the piston as well as the connecting rod. This piston attachment technique results in a very concentric piston. However, snap rings periodically become disengaged during operation as a result of improper installation resulting in serious damage if not a complete failure of the engine. The use of snap rings is also costly in that not only do snap rings need to be purchased and carefully installed, but snap ring grooves need to be machined in the piston requiring yet another machining operation.
Yet another method of securing the wrist pin to the piston which is frequently used in small engines is to stake the piston and/or end of the wrist pin so as to cause sufficient deformation to prevent relative longitudinal movement. Staking, while very economical, is not without problems. If a stake is placed with an adequate force, there is the risk that the wrist pin may become dislodged subsequently during engine operation. If the stake is achieved with too great a force, the piston may become out-of-round resulting in excessive piston wear and potentially premature failure.